1. Field of the Invention
This invention relates to plane wave rectangular waveguides with high impedance walls.
2. Description of the Related Art
New generations of communications, surveillance and radar equipment require substantial power from solid state amplifiers at frequencies above 30 gigahertz (GHz). Higher frequency signals can carry more information (bandwidth), allow for smaller antennas with very high gain and provide radar with improved resolution. However, amplifying signals with frequencies above 30 GHz using conventional methods does not provide optimal results.
At lower frequencies, available signal power can be increased by adding the output power of two or more amplifiers in a power combining network. For solid state amplifiers, as the frequency of the signal increases the size of the transistors within the amplifier devices decrease. This results in a corresponding reduction in the amplifier power output so that more amplifier devices are required to achieve the necessary power level. For instance, at millimeter wave frequencies the power per amplifier device for a set 10 dB gain ranges from 100 milliwatts (mW) at 30 GHz to 10 mW at 100 GHz. To attain power of more than a watt, at the higher frequencies, hundreds of amplifiers must be combined. This cannot be done by conventional power combining networks because of the insertion loss of the network transmission lines. As the number of amplifiers increases, a point will be reached at which the loss experienced by the transmission lines will exceed the gain produced by the amplifiers.
One method of amplifying high frequency signals is to combine the power output of many small amplifiers in an quasi-optic amplifier array. The amplifiers of the array are oriented in space such that the array can amplify a beam of energy rather than amplifying a signal guided by a transmission line. The amplifier array is referred to as quasi-optic because the dimensions of the array become more than one or two wavelengths. The beam of energy can be guided to the array by some form of a waveguide or the beam can be a Gaussian beam aimed at the array. {C. M. Liu et al, Monolithic 40 Ghz 670 mW HBT Grid Amplifier, (1996) IEEE MTT-S, p. 1123}.
Amplifier arrays can be produced as monolithic microwave integrated circuits (MMIC). In MMICs all interconnections and components, both active and passive, are fabricated simultaneously on a semiconductor substrate using conventional deposition and etching processes, thereby eliminating discrete components and wire bond interconnections. Quasi-optical amplifier arrays can combine the output power of hundreds of solid state amplifiers formed in a two-dimensional monolithic array on the plane normal to the input signal.
The primary method for guiding high frequency signals to an array amplifier uses a rectangular waveguide with conductive sidewalls. FIG. 1 shows a conventional metal waveguide 10 having four interior walls 11a, 11b, 11c, 11d. A signal source at one end 12 transmits a signal down the waveguide to a quasi-optical amplifier array mounted at the opposite end 13, normal to the waveguide. The numerous small amplifiers of the array amplify the signal and the combination of the amplifiers results in significant amplification of the signal. The E field orientation from the output of the amplifier will be orthogonal to the input E field orientation to reduce oscillatory instability. An output waveguide can be included to guide the output signal to a useful load. Using this method, results have been published showing an ability to reach substantial power at frequencies from 35 to 44 Ghz,{J. A. Higgins, Development of a Quasi-Optic Power Amplifier for Q Band, A Contract Final Report. Contract F30602-93-C-0188, USAF Rome Laboratory, 26 Electronic Parkway, Griffis AFB NY 13441.}
However, a rectangular waveguide with conductive sidewalls does not provide an optimal signal to drive an amplifier array. As shown in FIG. 2, a vertically polarized signal 21 has a vertical electric field component(E) 22, a perpendicular magnetic field component(H) 23, and a propagation axis (P). Because the sidewalls 11a and 11c of the metal waveguide of FIG. 1 are conductive, they present a short circuit to the E field. The E field cannot exist near the conductive sidewall and the power densities of both the E field 24 and the H field 26 drop off closer to the sidewall as shown in FIG. 2. As a result, the power density of the transmission signal 21 varies from a maximum at the middle of the waveguide to zero at the sidewalls 11a and 11c. If the waveguide cross-section were shaped to support a horizontally oriented signal, the same problem would exist only the signal would drop off near the top wall 11d and bottom wall 11b. 
For an amplifier array to operate efficiently, each individual amplifier in the array must be driven by the same power level, i.e. the power density must be uniform across the array. When amplifying the type of signal provided by the metal waveguide, the amplifiers at the center of the array will be overdriven before the edge amplifiers can be adequately driven. In addition, individual amplifiers in the array will see different source and load impedance depending upon their location in the array. The reduced power amplitude along with impedance mismatches at the input and output make most of the edge amplifiers ineffective. The net result is a significant reduction in the potential output power.
As an example of the power loss in conductive sidewall rectangular waveguide applications, measurements of a 1.2 cm by 1.2 cm array of 112 small amplifiers have provided an output power of 3.0 W at 38 Ghz. If a signal with uniform power density were applied to the same amplifier array the output power would be in excess of 10 W.
A high impedance surface will appear as an open circuit and the E field will not experience the drop-off associated with a conductive surface. A photonic crystal surface structure has been developed which exhibits a high wave impedance over a limited bandwidth. {D. Sievenpiper, High Impededance Electromagnetic Surfaces, (1999) PhD Thesis, University of California, Los Angeles}. The surface structure comprises xe2x80x9cthumbtacksxe2x80x9d of conductive material mounted in a sheet of dielectric material, with the pins of the thumbtacks forming conductive vias through the dielectric material to a continuous conductive layer on the opposite side of the dielectric material. This surface presents a high impedance to an incident EM wave but it has the characteristic of not allowing surface current flow in any direction. The gaps between the thumbtacks present an open circuit to any surface conduction.
Dielectric-loaded waveguides, so called hard-wall horns, have been shown to improve the uniformity of signal power density. {M. A. Ali, et.al., Analysis and Measurement of Hard Horn Feeds for the Excitation of quasi-Optical Amplifiers, (1998) IEEE MTT-S, pp. 1913-19211}. While an improvement in uniformity, this approach still does not provide optimal performance of an amplifier array in which input and output fields of a signal are cross polarized.
The present invention provides an improved high impedance surface structure used in waveguides which allows for the transmission of high frequency signals with a near uniform power density across the waveguide cross-section. The new sidewall surface provides a high impedance termination for the E field component of the signal flowing in the waveguide and also allows conduction down the other two walls to support the H field component of the signal. The power wave assumes the characteristics of a plane wave with a transverse electric and magnetic (TEM) instead of a transverse electric (TE) or transverse magnetic (TM) propagation. This transformation of the energy flow in the waveguide provides a wave similar to that of a free-space wave propagation having near uniform power density.
The new wall structure comprises a sheet of dielectric material with a conductive layer on one side. The opposite side of the dielectric material has a series of parallel conductive strips of uniform width, with uniform gaps between adjacent strips. Vias of conductive material are provided through the dielectric material between the conductive layer and the conductive strips. The actual dimensions of the surface structure will depend on the materials used and the signal frequency.
During transmission, the waveguide carries a signal having an E field component transverse to the surface structure""s conductive strips. At a resonant frequency the through substrate vias present an inductive reactance (2ΠfL) and the gaps between the strips present an equal capacitive reactance (1/(2ΠfC)). The surface presents parallel resonant L-C circuits to the transverse E field component; i.e. a high impedance. The L-C circuits present an open-circuit to the transverse E-field, allowing it to remain uniform across the waveguide.
Waveguides that transmit a signal in one polarity have the new wall structure on two opposing walls. For instance, a signal wave with a vertical polarity has a vertical E field component. A waveguide with the new surface structure mounted on the sidewalls (with the conductive strips oriented longitudinally) will present an open circuit to the E field at resonant frequency. The top and bottom walls remain conductive, which allows for a uniform H field.
In waveguides that transmit cross-polarized signals (both horizontal and vertical), the new wall structure is used for all four walls. The wall structure will present a high impedance to the transverse E field component of signal in both polarizations. The strips of the new wall structure also allow current to flow down the waveguide, which provides for a uniform H field in both polarizations. Thus, the new waveguide can maintain a cross-polarized signal with uniform density.